Before discovering fondant, Pastry Pride was all I used. Mix it about half-way, until it starts to get foamy and thicken up. Then add the instant (has to be instant) pudding mix. What is so great is that you can use any flavor. Then whip it until it's like fluffy frosting. A little hint, though, always whip on medium to medium/high speed. It takes a while but you will have less air pockets and it will be easier to smooth.

As far as leaving it out...you could probably leave it out a month with no problem. If you live where it is really hot and you have no AC, you might want to put it in the frig. One more little hint, if you cover the frosted cake immediately, the Pastry Pride wont form a "skin." If you leave it out a little while first, it forms the skin (almost like "crust" on a buttercream frosted cake. HTH